Thermal mass is the ability of a material to absorb and store heat. It takes a lot of heat to raise the temperature of a dense material like concrete, so this has high thermal mass. It takes much less to raise the heat in light materials like timber, so these materials have low thermal mass.

For thermal mass to be effective it needs to absorb heat when it is available, store it until needed and release that heat when appropriate. This is when things can get complicated. Use the wrong materials or put them in the wrong place and they won’t help at all with heating or cooling.

How Can Thermal Mass Work for My Project?

There are two basic ways of dealing with thermal mass: you can design it into your scheme or you can simply let it happen.

Design in Thermal Mass

This involves, among other things:

Y values (W/m2K)

Psi-values (W/mK)

thermal capacity of the materials

the density of the materials

thermal lag (the time it takes for that material to release its stored heat, usually measured in hours)

orientation of the building

glass-to-floor ratios.

It’s a complicated process that is particularly important when thermal mass is a feature of a lightweight construction such as timber frame.

Let Thermal Mass Happen

Thermal mass has been an integral part of housebuilding for centuries, and still is in many parts of the world.

In southern Europe and other hot regions of the world, thermal mass in the form of large volumes of masonry, concrete and adobe is still the ‘go-to’ building option. Because it absorbs heat, it maintains a lower internal temperature and evens out temperature differences, day to night. In these instances, thermal mass is not designed in, it just happens.

In this country, the material of choice for many old cottages is stone. It is a good, robust, cheap material and, by chance, provides thermal mass. Here, the problem is not so much keeping heat out (but in warm weather it has that effect) and more to do with storing and releasing the heat produced naturally inside. In previous centuries there would have been a fire going day and night (for cooking and boiling water as well as heating) that allowed the walls and floor to be warmed by the fire all day (all year) and kept the house warm overnight.

We no longer live in that way, and relying on heat from our boiler to warm the thermal mass may not be the best idea. The thermal mass will even out the peaks and troughs of the heating cycle, but does little else.

The Importance of the Heat Source

Thermal mass needs to absorb heat to be effective. If that heat is free or cheap from solar energy coming through a window or a woodburning stove, for example, then all is well.

If we rely on a boiler to provide the heat, then that heat has to be paid for and the requirement is to heat the house and the thermal mass. This will smooth the peaks and troughs in the heating cycle and help with summer overheating, but there is no evidence that it will reduce the overall energy consumption or the fuel bill.

Does Thermal Mass Equal Heavyweight Buildings?

In recent years, thermal mass has been taken up as a way of ‘improving’ energy efficiency. In 2005 Arups published the report UK Housing and Climate Change. The report argued that rising temperatures could bring a Mediterranean climate to the UK, and that the inclusion of thermal mass was a way of countering this possible future problem.

The argument was taken up by the manufacturing sector to justify heavyweight construction. However, this ignored the fact that timber frame and SIPs (structural insulated panels) can also effectively incorporate thermal mass.

Thermal mass is not a ‘one-size-fits-all’ idea, although the housebuilding industry still uses it to justify brick and block construction.

Thermal Mass Materials

The thermal mass performance – or volumetric heat capacity (VHC) – of materials varies widely. In this list, concrete has four times as much thermal mass as aerated concrete blocks:

Concrete 2,060

Sandstone 1,800

Compressed earth blocks 1,740

Rammed earth 1,673

Brick 1,360

Adobe 1,300

Aerated concrete block 550

The VHC of any material is reduced or even eliminated if the material is covered with linings such as carpets, plasterboard and timber. Some manufactured materials, such as concrete, bricks and blocks, have high embodied energy and you could say that the lifetime energy impact of the material outweighs the savings in heating energy that it brings.

In addition, thermal mass materials are inherently cool (which is why they are so common in hot climates) and in our climate need heating. Poor design of thermal mass can result in increased heating energy demand to maintain a comfortable internal temperature, as the need is then to heat the house and the thermal mass.

Phase Change Materials

There is a growing interest in phase change materials (PCMs) as a lightweight thermal mass substitute in construction. Phase change refers to materials changing their state with the application of heat. For example, when water is heated it changes from solid (ice) to liquid (water) to gas (water vapour).

Manufacturers suggest that a 13mm plasterboard with 30 per cent PCM content has the equivalent heat capacity of a 150mm thick masonry wall.

Conclusion

Climate change and the increasing use of glazing means that overheating is becoming a real issue and we know that thermal mass is an effective solution.

We all understand that a dark, matt surface in front of a window will absorb more heat than a white shiny surface. We also understand that cooler air falls, pushing the warm air up.

At its most basic level, thermal mass is no more complicated than that. PCM boards on the ceiling of a room with lots of south-facing glazing will be effective, as will a matt stone floor in front of that glazing and a stone chimney on the wall opposite the glazing.

‘Designed-in’ thermal mass requires a deep understanding of the materials and what can realistically be achieved. In our climate, it also requires the house to be designed around the concept of thermal mass. This means designing to introduce large quantities of passive solar heat and using thermal mass to store and release that heat at the appropriate times.

The ‘let-it-happen’ approach to thermal mass can have a negative impact if it is done poorly. But if thermal mass is not the main design criterion, using a little intelligence makes it very difficult to get wrong.

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